The invention relates to a device for comparing nominal wavelength values and wavelength values of optical signals coupled with the input ports of a multiplexer with a waveguide grating, the signals carried by the different wavelengths combining at an output port of the multiplexer.
The invention also relates to a system for monitoring monochromatic optical sources, serving notably to generate the signal carrier waves.
The comparison is made with a view to controlling the wavelength value of the optical sources used in order to effect wavelength multiplexing in telecommunications networks. The increase in density requirements of transmission channels in networks using wavelength multiplexing accentuates the importance of rapid control of the wavelengths of monochromatic sources and their stability; these wavelengths can undergo drift due to aging or variations in temperature of the monochromatic sources, for which generally lasers would be used. Nominal wavelength means the emission wavelength imposed on the laser. These nominal wavelengths correspond typically to standard values. Input port (or output port) generally means an input (or output) guide.
Amongst the different types of passive multiplexers known, consideration will be given hereinafter to those which use an angular dispersion element such as an etched diffraction grating or a grating formed by waveguides, connecting two couplers in a star. Hereinafter, the term phasar will be given to this type of multiplexer (referred to in English as xe2x80x9cPhased Arrayed Waveguide Grating Multiplexerxe2x80x9d or xe2x80x9cAWGxe2x80x9d).
An example of a wavelength control device for the optical emitters of a phasar is proposed in the article xe2x80x9cA wavelength matching scheme for multiwavelength optical links and networks using grating demultiplexersxe2x80x9d by F. Tong et al, IEEE Photonics Technology Letters, vol 7, No 6, June 1995. In the solution proposed, two transmission channels are dedicated to the comparison of the wavelengths.
In the article xe2x80x9cFabrication of multiwavelength simultaneous monitoring device using arrayed-waveguide gratingxe2x80x9d, Electronics Letters, 14 March 1996, Vol. 32, No 6, the authors K. Okamoto et al describe a device making it necessary to take off some of the N input signals in order to reintroduce them into the phasar and compare at the output the corresponding N signals detected on each side of the N main output signals. In addition to a few losses caused by taking off some of the N input signals, this solution makes it necessary to duplicate the input signals, an operation which is tedious to perform.
The aim of the invention is to propose a solution not having the drawbacks mentioned above.
A detailed analysis of the functioning of the phasar type envisaged in the invention and depicted in FIG. 1 shows that, for each wavelength value xcexi of the emission laser i, i varying from 1 to N, the optical wave constituting the input signal is coupled with an input guide GEi and undergoes the following operations:
a diffraction in an input coupler CE, each guide Gj situated at the output surface SCE of the coupler receiving part of the diffracted wave,
phase shifts in a grating R of M guides Gj, j varying from 0 to M, situated between the coupler CE and a coupler CS, the length of the guides Gj varying according to a linear function such that LGj=LGO+jxcex94L, xcex94L being the difference in length between two adjacent guides; the phase differences forming, at the output of the grating R of guides, interferences which are constructive in a direction depending on the wavelength,
a focusing, on the output surface SCS of the coupler CS, of the interferences of the waves issuing from the guides of the grating R.
The phasar according to the invention is designed so that the adjacent interference orders mxe2x88x921, m and m+1 correspond to three focal points which form on the output surfaces SCS of the coupler CS for N given wavelengths, that is to say for the N wavelengths of the lasers used. The multiplexed signals resulting from the interferences at orders mxe2x88x921 and m+1 of the grating R are used for the wavelength control; whilst the multiplexed signals resulting from the interferences at order m have the maximum power and correspond to the useful signal.
More precisely, the object of the invention is a device for comparing between N nominal wavelength values xcexei and the wavelength values of optical signals coupled respectively with input ports GEi of a phasar with a grating R of guides Gj, the said phasar comprising a main output port GS(m) designed to collect the combination of the said signals, the said main output port GS(m) being placed at a focal point corresponding to a given interference order m of the grating, principally characterised in that the phasar comprises two secondary monitoring output ports (A, B) respectively placed in the vicinity of focal points corresponding to the interference orders mxe2x88x921 and m+1 of the grating R.
Thus, with a single device, it is possible to fulfil the wavelength multiplexing function and a function of monitoring the wavelengths of the emission sources of the signals to be multiplexed. The proposed solution does not require duplication of the input signals. In addition, as the focal points for the orders mxe2x88x921 and m+1 exist in any event in the usual phasars, they do not cause any additional loss.
Concerning the practical embodiment, an appropriate choice of the positions of the monitoring ports makes it possible to optimise the monitoring function. For this, the transmission functions TAi and TBi of the phasar defined respectively as the ratios of the optical powers present respectively at the outputs A and B to the optical power of the signal applied at the input port GEi, these power ratios being a function of the wavelength of the signal applied.
Thus, according to a characteristic of an embodiment of the invention, the positions of the secondary output ports A and B are adjusted so that the difference between the two transmission functions of the phasar TAi and TBi for the output ports A and B, called the discrimination function Di, is zero for all the nominal wavelength values xcexei and bijective as a function of the wavelength value around the nominal value of the wavelength value most dispersed by the grating at orders mxe2x88x921 and m+1. The bijective character means that, to each wavelength value taken around this nominal value, there corresponds a single value of the discrimination function Di and vice-versa.
The positions of the secondary output ports A and B can be adjusted so that, for all the wavelength values xcexei, the spectral difference xcex94xcexei between the wavelength value of the maximum of the transmission curve TAi and the wavelength value of the transmission curve TBi is situated at the upper limit of a minimum spectral difference xcex94xcexc determined experimentally. This condition ensures the bijective character of the discrimination function Di.
To allow electronic processing of the function Di, the two secondary output ports are coupled with photodetectors.
The invention also relates to a system for monitoring monochromatic sources Li, characterised in that it includes a comparison device as described above, whose input ports GEi are coupled respectively with the said sources, and control means CM for monitoring the wavelength of at least one of the said sources according to the electrical signals supplied by the said photodetectors.